This invention relates generally to Chemical Vapor Deposition (CVD) Systems. More specifically, this invention relates to a method and apparatus for reducing gas flow turbulence within an epitaxial CVD reaction chamber of a CVD system, thereby improving deposition quality onto a substrate.
CVD is the formation of a solid compound on a heated substrate by thermal reaction and/or decomposition of gaseous compounds. Epitaxial deposition is a specific type of CVD where a crystallographic orientation of the substrate is replicated in the growing film. Prior art CVD systems incorporate many variations in the number and type of components used for implementation of the deposition process, but they generally include a reaction chamber which contains the wafer or substrate, a gas control system, an electronic control system, a heat source, and a waste treatment system for disposing of exhaust gases.
The key to an efficient reaction chamber is to provide a controlled environment that allows for a safe and stable deposition of compounds. The chamber may be of any suitable material, but quartz is typically employed because its inert properties prevent chemical reactions between it and the deposition gases. Epitaxial deposition reactors can be classified into three general designs, namely, horizontal, vertical, and cylindrical systems. In horizontal systems, wafers are placed horizontally on boats or susceptors. Horizontal systems are configured to allow the gas to flow into the reaction chamber from one end, across the wafers, and out of the reaction chamber at an opposite end. Vertical systems have the wafers placed horizontally on a susceptor mounted rotatably within the chamber, and the gas flows vertically downward from the top of the reaction chamber towards the wafers to ensure a uniform temperature and gas distribution. Cylindrical or barrel reactors have the wafers removably mounted onto the outer surface of a rotatable cylindrical susceptor. The gases in a cylindrical system flow vertically downward into the reaction chamber from the top, passing over the wafers, and then exit the reaction chamber through an exhaust at the bottom.
A heating process of a cold wall CVD reactor is needed to facilitate the desired chemical reaction between the gas and the wafer or substrate. The heating process is accomplished by means of radio frequency (RF) energy, radiation energy in the ultraviolet (UV), visible, or infrared (IR) bands, or resistance heating. Susceptors heated by the RF method are typically made of silicon carbide coated carbon and employ energy coupling through an RF coil to channel the energy of the RF coil into the susceptor. Heating of the wafers is achieved by contact with the susceptor. UV or IR heating is achieved using high intensity lamps that emit light energy in the ultraviolet, visible, and/or infrared spectrum. The radiation energy emitted from such lamps heats the wafer and the susceptor. In cold wall reactors, the walls of the chambers are kept cool compared to the wafer to prevent leakage of radiation into the exterior of the system.
Prior to the heating process, any residual air remaining within the reaction chamber needs to be removed or purged. Similarly, any process gases remaining in the chamber after the processing and cool-down processes must be flushed out. Purge gases typically comprise non-reactive gases such as nitrogen and are used at the beginning and the end of each deposition cycle. Purging is done prior to opening the chamber after a complete deposition process in order to assure that no reactive component gases are left within the reaction chamber.
The deposition process itself involves the transportation of reaction gases containing the necessary chemical components across the wafer such that a film can be grown onto the wafer. The deposition process also simultaneously provides for the doping of the growing film. Process gases can include gaseous components that deposit, dope, and etch depending on the desired process flow. All such gaseous components are transported through the reaction chamber by means of a carrier gas which is typically hydrogen or helium. The carrier gas is used before, during and after the deposition cycle. Etching gases, such as anhydrous hydrogen chloride (HCl) may be used before the actual deposition cycle to remove a thin layer of silicon, thereby creating a hydrophobic surface and removing any foreign matter or crystallographic defects prior to the film deposition. Once the process is initiated, the chamber flushed, and the right temperature achieved, the process gases are added to the carrier gas. Conventional source gases for silicon deposition are Silane (SiH.sub.4), Dichlorosilane (SiH.sub.2 Cl.sub.2), Trichlorosilane (SiHCl.sub.3) or Silicon Tetrachloride (SiCl.sub.4). Dopant gases added to the gas components during film growth normally comprise Arsine (AsH.sub.3), Phosphine (PH.sub.3) or Diborane (B.sub.2 H.sub.6).
Unfortunately, most prior art systems for CVD inherently produce a non-uniform deposition on the wafer surface, partially due to the presence of particles, contaminants, wall deposits, and other defects within the reaction chamber. All of these defects can cause particle generation, inhomogeneous gas flow over the wafer, and low productivity. Non-uniform depositions can also be caused by uncontrolled gas velocity or density profiles. The industry has traditionally been unable to maintain quality in conventional CVD systems for depositing silicon onto a substrate because conventional reaction chambers used in these systems have a non-uniform cross-sectional area through which the deposition process gases flow. Gas flow turbulence results as those gases encounter variations in cross-sectional area and in the surface features of the chamber walls. Furthermore, Light Point Defects (LPDs) result as settled particles are disturbed from the floor of the reaction chamber by the gas flow turbulence and then find their way to the substrate surface. In addition to turbulence, non-uniformity of cross-sectional area in the reaction chamber results in variations in the velocity of the gas flow through the reaction chamber which also adversely affects the quality of the deposition.
These problems have become even more significant with the advent of single wafer processing systems, especially for large diameter wafers for future device processes. Compared with batch type systems, continuous flow single wafer systems require a rapid deposition rate to minimize process time, reduce gas usage, and maximize productivity. A high through-put with minimal gas usage, minimized particle generation, and uniform deposition rates over the wafer surface are the primary objects of efficient single wafer epitaxial CVD processing. However, all of these requirements are highly challenging to achieve.
To more clearly understand the process and apparatus for single wafer epitaxial CVD deposition, the following explanation of the prior art is provided with reference to FIG. 1. A typical single wafer epitaxial reaction chamber has a top panel 1, a bottom panel 2, a vertical wall section 3, a lower bottom panel 4, a gas inlet flange 5, a gas injector 6, a gas outlet flange 9, a gas outlet 11, a quartz susceptor support 12, a susceptor 13, a rotary shaft 15, and a shaft outlet 16. The gas enters a front portion 18 of the reaction chamber via the gas injector 6. A resulting gas flow and velocity are symbolized by the arrow 7. The gas flow 7 travels across the wafer 14 and proceeds into the rear portion 10 of the reaction chamber. The wafer 14 is removably positioned on the susceptor 13. The susceptor 13 is mounted onto a quartz susceptor support 12 connected to the rotary shaft 15. The quartz susceptor support 12, and hence the susceptor 13 and wafer 14, are typically rotated during film deposition by means of the rotary shaft 15. The rotary shaft 15 is fitted into a shaft outlet 16, such that the rotary shaft 15 can be moved vertically to raise or lower the susceptor 13 within the reaction chamber. It should be understood that variations in operational features can be employed, such as, placing three evenly spaced wafers on the susceptor 13, for example.
A general problem with the prior art is the fact that the lower portion 8 tends to collect particles along the lower bottom panel 4 that may migrate to the wafer surface 14a as a result of gas turbulence. Such turbulence is unavoidable when a gas flow encounters irregularities in the reaction chamber such as corners, protrusions, or a change in the cross-sectional area.
As can be seen in FIGS. 1, 2A and 2B, the typical single wafer epitaxial reaction chamber has several such irregularities which will disturb a homogenous gas flow.
Referring to FIGS. 1 and 2B, the gas flow 7 passes over the bottom panel 2 and the wafer 14 before entering the rear portion 10 of the reaction chamber where it encounters an increase in cross-sectional area at the interface between the susceptor 13 and the rear portion 10. FIG. 2A shows a top view of said single wafer epitaxial reaction chamber with the top panel 1 removed. The width of the reaction chamber begins to radially expand at the point of gas entry from the gas injector 6 to approximately the center of the susceptor 13. The width then radially decreases in the rear portion 10, from about the center of the susceptor 13 to the gas outlet 11. As is inherent in such designs, both the horizontal and the vertical changes in cross-sectional area create gas flow irregularities. FIG. 2C illustrates the variations in the gas flow velocity v (mm/s) over the distance x (mm) of the reaction chamber.
The gas flow velocity v decreases 20 as the cross-sectional area increases, but slightly increases 21 where the chamber begins to narrow. However, the gas flow velocity v abruptly drops 22 at the end of the susceptor 13 due to the sudden increase in cross-sectional area, and finally increases again 23 due to the narrowing of the cross-sectional area of the rear portion 10. These gas flow disturbances cause turbulence within the reaction chamber which detrimentally affects epitaxial layer thickness control and increases the LPD density caused by undesired particles reaching the wafer surface 14 a during deposition.
U.S. Pat. Nos. 5,096,534; 5,244,694; 5,525,157; and 5,411,590 disclose single wafer reaction chamber designs. Each of these patents describes reaction chambers having rectangular cross-sectional areas with improved gas flow control. These patents also teach that such a chamber design helps to achieve a more uniform deposition on the wafer surface, to minimize the presence of particles and contaminants within the reaction chamber, to reduce wall deposits on the interior of the reaction chamber which can cause particle generation, to achieve homogenous gas flow over the wafer to be processed, and to realize higher productivity by controlling gas velocity and density profiles. None of these patents, however, deals with or suggests how to retrofit existing reaction chambers having non-uniform cross-sectional areas in order to provide these same desirable attributes. Replacing the entire reaction chamber can be tedious and difficult, and is more expensive and time consuming than retrofitting an existing reaction chamber.
What is needed, therefore, is a means for configuring an existing reaction chamber, having a non-uniform cross-sectional area, such that gas turbulence and velocity variations are reduced or minimized.